Load following single bed reversing blower adsorption air separation system

ABSTRACT

An exemplary single bed reversing blower adsorption based air separation unit is configured to follow the O 2  load placed thereon by adjusting flow rates therethrough and power consumption. At least one and preferably multiple pressure sensors sense O 2  pressure within an O 2  storage region downstream of an adsorber vessel. These sensed pressures are utilized to generate control signals controlling flow rates at locations upstream of the compressor, such as at a reversible blower and an output compressor. Control loops for the blower and the compressor are independent of each other and have different time constants. Effective following of the O 2  load is thus achieved without driving the air separation unit into operational conditions outside of design and also maintaining optimal power consumption for the O 2  produced, such that efficiency is maintained over a large turndown ratio.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit under Title 35, United States Code§119(e) of U.S. Provisional Application No. 62/098,052 filed on Dec. 30,2014.

FIELD OF THE INVENTION

The following invention relates to air separation units for separationof oxygen from air, such as utilizing a single bed reversing bloweradsorption based air separation unit. More particularly, this inventionrelates to single bed reversing blower adsorption based air separationunits which can have an oxygen supply therefrom efficiently adjusted tomatch an oxygen demand from the air separation unit.

BACKGROUND OF THE INVENTION

The production of oxygen using vacuum swing adsorption (VSA) iswell-known to air separation technologists. VSA offers a simplenon-cryogenic method to produce gaseous oxygen at purities of 80% to95%. In the last 20 years oxygen VSA plants have become widespread andare offered in various bed configurations. The multi-bed VSA istypically used in the size rage of 60 tons per day (TPD) and higher. Thesingle bed process was adopted as a lower capital, simpler process forlower production ranges, typically 1 TPD up to 40 TPD. Typical singlebed systems usually consist of a single blower train that is used forboth the feed air provider as well as the regeneration vacuum system.The process usually incorporates automatic valves to direct the air andvacuum flows during the cycle. A newer embodiment of the single bedprocess uses a reversing blower to generate the feed stream and applyvacuum for the regeneration step. This latest embodiment is well suitedfor small to medium sized oxygen VSA production plants (1 to 10 TPD).One example of a single bed reversing blower (SBRB) VSA process of thistype is described in U.S. Pat. No. 8,496,738.

Although the single bed reversing blower (SBRB) VSA process is simple inpractice, its simplicity comes with performance trade-offs when comparedto multi-bed systems. Firstly, the lack of additional adsorber beds doesnot allow for a crucial bed to bed equalization. The pressureequalization step is key to lowering power consumption and increasingproduct oxygen recovery. Technologists in the art have overcome thisdeficiency by adding an equalization tank to the SBRB system (such asequalization tanks in SBRB systems provided by Air Liquide of Houston,Tex.).

Another waste of power in any adsorption process is the poor turndownratio of the process. In a multi-bed, dual blower system turndown islimited by the fact that the blowers must remain powered up. One optionfor turndown in such systems consists of running the feed blowerdischarge into the vacuum system suction piping, in effect shortcircuiting the VSA process. While not particularly efficient, theblowers are allowed to run unloaded, saving some power.

SUMMARY OF THE INVENTION

In a single bed system one can use variable frequency drives for themotor to achieve a significant and efficient turndown ratio, typicallydown to twenty-five percent to ten percent of name plate. However, it isdifficult to construct an algorithm to control the variable frequencydrive appropriately to achieve the efficient turndown ratio desired. Toprovide such a control algorithm according to this invention, a productbuffer storage tank located downstream of the adsorber vessel monitorspressure levels therein. Adsorption equilibrium is dynamically managedby adjusting the inlet flow rate into the adsorption vessel according tofeedback received from the pressure levels within the buffer storagetank.

Following such an algorithm, and with inputs of pressure from theproduct buffer storage tank providing control of an inlet flow rate intothe adsorber bed, vessel flow rates and velocities far below designcenter are provided which still maintain high purity and adsorptionequilibrium with lower power required and with lower product gas(typically oxygen) output. Such a system can further include controlfrom an operator selecting an amount of turndown to be provided, causingthe system to operate closer or further away from design center, butstill maintaining adsorption equilibrium and product gas purity, andwith lower power draw.

In an optimized embodiment of this invention, pressure is sensed at twolocations and oxygen production rates and power required for such oxygenproduction are controlled by the two sensed pressures. A first pressureis sensed at a first location which is preferably at an O₂ buffer tank(also called an O₂ process tank) within an O₂ storage area downstream ofthe adsorption bed. This first pressure is utilized to control a flowrate through the reversible blower. In particular, if the sensedpressure at the first location is greater than a pressure set point,indicative of overpressure and slack demand for oxygen, flow ratethrough the reversible blower is reduced. Specifically, the blower isdriven at a slower speed so that its flow rate therethrough (whetherforward or rearward) is reduced. In one embodiment, the blower is drivenby a variable frequency drive which can be readily controlled to achievea lower flow rate therethrough. At such a lower flow rate, oxygen isproduced more slowly by passing air more slowly through the adsorptionbed. Oxygen thus flows on into the buffer tank more slowly, tending todecrease pressure therein. If, alternatively, the pressure sensed atthis first location is decreasing, such a decreasing pressure isindicative of increased demand for oxygen from the system. Such adecreased pressure is fed back through a controller to the reversibleblower to increase a flow rate through the reversible blower.

Additionally, pressure is preferably sensed at a second location in apreferred form of the invention. The second pressure is senseddownstream of a compressor which is downstream of the buffer tank andsupplies pressurized oxygen from the system. The second pressure sensedat the second location downstream of the compressor is compared to a setpoint. If this second pressure is sensed to be higher than the setpoint, this is indicative of slack O₂ demand and is fed back to thecompressor to cause the compressor to operate at a lower power and lowerflow rate therethrough. If, alternatively, the pressure is sensed to belower than the set point at the second location, this is indicative ofheightened demand and the compressor is driven at a higher flow ratewith greater corresponding power consumption. In this way, thecompressor is also acting responsive to oxygen demand.

These two separate feedback loops both sense pressure and feedback acontrol signal to increase or decrease flow rate through upstreamequipment. To keep the equipment operating in harmony with each otherand avoiding circumstances where the two control systems respond to eachother rather than to actual O₂ load/demand, a time interval at which thetwo control systems pause between sensing pressure and adjusting flowrates associated with each control loop are set to be distinct from eachother. In one embodiment, the compressor control system senses pressureapproximately every second and responds appropriately. At the same time,the buffer tank pressure sensed at the first location is allowed toadjust reversible blower flow rate approximately every minute.

The presence of the buffer tank and the volume thereof ensures thatrapidly increasing demand sensed at the second location can be respondedto by the compressor and the compressor will not exhaust the supplywithin the buffer tank before the heightened demand is sensed andresponded to at the first location within the buffer tank, causing thereversible blower to increase flow rate and produce oxygen more quickly.Correspondingly, if oxygen demand sensed by the pressure sensor at thesecond location decreases rapidly, causing the compressor to operate ata significantly lower flow rate, the buffer tank is sufficiently largethat the reversible blower can continue at a high flow rate (forapproximately a minute) to produce oxygen which can be stored up in thebuffer tank, with such oversupply being readily absorbed within thebuffer tank for the up to one minute time period that such a rapidlydecreasing demand might cause, before the sensed increased pressure inthe buffer tank causes a reduction in the flow rate of the reversibleblower. Stable load following is thus achieved and efficient oxygenproduction is maintained even with a turned down ratio of fifty percent,seventy-five percent or more.

OBJECTS OF THE INVENTION

Accordingly, a primary object of the present invention is to provide asingle bed reversing blower (SBRB) vacuum swing adsorption (VSA) orother adsorption based air separation unit which can flexibly meetdemand over a wide range, by effectively matching O₂ demand with flowrates within the air separation unit.

Another object of the present invention is to provide a single bedreversing blower air separation unit which maintains efficient operationeven when O₂ demand varies widely.

Another object of the present invention is to provide an SBRB VSA airseparation unit which can maintain continuous operation at variable flowrates and avoid excessive start up and shut down cycles, while meetingvariable O₂ demand.

Another object of the present invention is to provide an air separationunit which operates in a manner following the load placed on the airseparation unit with a high turn down ratio while still maintainingefficient O₂ producing operation.

Another object of the present invention is to provide a method forcontrolling flow rates of equipment within an adsorption based airseparation unit to efficiently respond to changes in O₂ load.

Other further objects of the present invention will become apparent froma careful reading of the included drawing figures, the claims anddetailed description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a prior art single bed reversing blower vacuumswing adsorption air separation unit typical for which the technology ofthis invention is addressed.

FIG. 2 is a schematic of a single bed reversing blower vacuum swingadsorption air separation unit incorporating a purge recovery tanktherein to enhance performance of the air separation unit and defining amodified vacuum swing adsorption air separation process, suitable forimplementation of this invention.

FIGS. 3-5 are schematics similar to that which is shown in FIG. 2, butwith various different arrows depicting various steps in the operationof the reversing blower vacuum swing adsorption air separation unit.FIG. 5 also includes a control system implemented by a controller andmultiple sensors and control outputs, to facilitate load following forthe air separation unit according to this invention.

FIG. 6 is a graph of power level response to sensed pressureillustrating how pressure sensed at first and second sensor locationsdepicted in FIG. 5 cause changes in power consumption of a drive of areversible blower and a drive of a compressor, for automatic andreliable O₂ demand load following according to this invention.

FIG. 7 is a graph of relative power consumption versus O₂ production,illustrating how air separation unit production efficiency is generallymaintained even with a turndown ratio of seventy-five percent or more.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawings, wherein like reference numerals representlike parts throughout the various drawing figures, reference numeral 10(FIG. 1) is directed to a prior art oxygen separator configured toseparate/concentrate oxygen from air. This separator is modified by theair separation unit 110 of this invention (FIGS. 2-5) and the blowerdriving system 210 of this invention (FIG. 4), as explained in detailbelow.

In essence, and with particular reference to FIG. 1, basic details ofthe oxygen separator 10 modified by the air separation unit 110 andblower driving system 210 of this invention are disclosed. The oxygenseparator 10 includes an adsorber bed 20 including an adsorber materialtherein which preferentially adsorbs nitrogen, CO₂ and water overoxygen. A valve 30 is located downstream of the adsorber bed 20. Abuffer tank 40 is provided downstream of the valve 30. A blower 50defines a preferred form of pump located upstream of the adsorber bed20. A controller 60 is coupled to the valve 30 and the blower 50 (orother pump) to control opening and closing of the valve 30 and tocontrol a direction in which the blower 50 (or other pump) is operating,to either provide air into the adsorber bed 20 or pull a vacuum todesorb and remove nitrogen out of the adsorber bed 20. Normally, a heatexchanger is required between the blower and the adsorber bed to removethe heat generated when the air is compressed. The heat exchanger may bebypassed during the vacuum phase of the cycle.

With continuing reference to FIG. 1, details of the adsorber bed 20 aredescribed. The adsorber bed 20 includes an enclosure 22 for containingthe adsorber material. This enclosure 22 includes an inlet 24 spacedfrom an outlet 26. The inlet 24 and outlet 26 define preferred forms offirst and second ports for access into the enclosure 22. The inlet 24and outlet 26 normally are incorporated in closures or “end plates”which can be removed to allow access to the adsorption components in theenclosure 22. Otherwise, the enclosure 22 is preferably sealed toprevent leakage of gases into or out of the enclosure 22.

The adsorber material within the adsorber bed 20 could be any form ofmaterial which preferentially adsorbs nitrogen over oxygen. One suchmaterial is molecular sieve such as nitroxy siliporite. This material ispreferably supplied in the form of beads which are either generallyspherical in form or can be of irregular shape. Since the beads arecomposed of molecular sieve material within the enclosure 22, gaseouspathways extend through, between and around the adsorbent material.

Most preferably, a plenum is configured at the inlet and the outlet endof the adsorber bed to provide even gas flow across the cross section ofthe bed. In a preferred configuration, the inlet 24 is located below theoutlet 26, and with the inlet 24 at a lowermost portion of the enclosure22 and the outlet 26 on an uppermost portion of the enclosure 22. Theenclosure 22 could have a variety of different shapes. In oneembodiment, the enclosure 22 could be generally rectangularly shaped.The enclosure could be shaped like a pressure vessel to maximize anamount of vacuum to be drawn on the enclosure 22 while minimizing anamount of material strength (i.e. wall thickness or material choice)that must be designed into the enclosure 22. If the size of the adsorbermaterial is sufficiently small to potentially pass through the inlet 24or outlet 26, filters are provided at the inlet 24 and outlet 26 to keepthe adsorbent material within the enclosure 22.

With continuing reference to FIG. 1, details of the valve 30 aredescribed. The valve 30 is interposed on a line 32 extending from theoutlet 26 of the adsorber bed 20 and extending to the buffer tank 40.This line 32 is preferably substantially rigid, especially between thevalve 30 and adsorber bed 20, so that when a vacuum is drawn on theadsorber bed 20, the line 32 does not collapse. The valve 30 ispreferably sealed to prevent leakage in any manner when in a closedposition and to only allow passage of gas along the line 32 when in anopen position.

The valve 30 is preferably coupled to a controller 60 which controls theopening and closing of the valve 30. Optionally, the valve 30 could havea controller built into the valve 30 that could be set a single time andthen operate in accordance with its settings.

While the valve 30 would typically be programmed once and then operatein accordance with such settings, the valve 30 could optionally becontrolled at least partially through a control system including sensorsand feedback to the valve 30. For instance, an oxygen sensor could beprovided adjacent the valve 30 or along the line 32 between the valve 30and the adsorber bed 20 to detect oxygen concentration levelsapproaching the valve 30. Nitrogen adjacent the valve 30 would beindicative that the adsorbent material within the adsorber bed 30 issaturated with nitrogen and that the oxygen separator 10 needs to changeoperating modes, to have the blower 50 (or other pump) reverse to pull avacuum and desorb nitrogen from the adsorber material and pull thenitrogen out of the adsorber bed 20 to recharge the system.

Normally control of the cycle is achieved with the use of pressuretransducers which reverse the blower at appropriate times. Usually thepurge cycle is initiated when the vacuum reaches a certain predeterminedlevel. The valve 30 is then opened for a predetermined amount of time sothat a purge layer of oxygen is allowed to purge the remaining nitrogenfrom the bed. So the pressure and vacuum cycle are determined bypressure and the purge portion of the cycle is timed.

Other sensors could also potentially be utilized to allow the oxygenseparator 10 to operate most effectively. The valve 30 is preferably ofa type which operates with a minimum of lubricant or which can operatewith a lubricant which is compatible with the handling of oxygen. Thevalve 30 and other portions of the oxygen separator 10 are alsopreferably formed of materials which are compatible with the handling ofoxygen. For instance, brass is often effective in handling of oxygen andso brass is one material from which the valve 30 could be suitablymanufactured when the system 10 is used for oxygen separation.

With continuing reference to FIG. 1, details of the buffer tank 40 aredescribed. The buffer tank 40 is not strictly required for operation ofthe system, but allows for the system in the form of the oxygenseparator 10 to deliver oxygen substantially continuously, and tomoderate pressure spikes in the system. The buffer tank 40 includes anenclosure 42 with an input 44 and an output 46 in FIG. 1. However,normally the buffer tank does not have a separate inlet and outlet.Since its purpose is simply to be an accumulator and minimize thepressure fluctuations inherent in the pressure swing adsorption process.The input 44 is coupled to the line 32 on a side of the valve 30downstream from the adsorber bed 20.

The buffer tank 40 would typically have some form of regulator valve onthe output 46 which would deliver oxygen out of the buffer tank 40 whenoxygen is required by oxygen utilizing systems downstream of the buffertank 40. The input 44 of the buffer tank 40 can remain in fluidcommunication with the valve 30. The buffer tank 40 can contain oxygenat above atmospheric pressure and at a pressure matching or slightlybelow an operating pressure of the adsorber bed 20 when the adsorber bed20 is actively adsorbing nitrogen and oxygen flows into the buffer tank40.

A sensor can be associated with the buffer tank 40 which cooperates withthe controller 60 to shut off the oxygen separator 10 when the buffertank 40 nears a full condition. In many applications a compressor islocated downstream from the buffer tank 40 to fill oxygen vessels. Whenthe vessels are full the system would be shut off. If required, apressure regulator can also be provided on the output 46 of the buffertank 40 so that pressure of oxygen supplied out of the buffer tank 40remains substantially constant. Similarly, an oxygen pump could beprovided downstream of the buffer tank 40 if the oxygen were required tobe supplied at an elevated pressure above pressure within the buffertank 40.

Most preferably, the buffer tank 40 is not a particularly high pressuretank so that the oxygen separator 10 including the blower 50 (or otherpump) and adsorber bed 20 do not need to operate at a particularly highpressure when delivering oxygen to the buffer tank 40. By minimizing thepressure of the buffer tank 40, the weight of the buffer tank 40 (andother components of the system 10) can be significantly reduced.Furthermore, the power consumed by the blower is reduced as the pressuredrop across the blower is reduced.

With continuing reference to FIG. 1, details of the blower 50 (or otherpump) are described. This blower 50 generally includes a housing 52 withsome form of prime mover therein coupled to a driver, such as anelectric motor. The housing 52 of the blower 50 includes an entry 54 indirect access with a surrounding environment in a preferred embodiment.A discharge 56 is also provided on the housing 52 which is located on aside of the blower 50 closest to the adsorber bed 20.

The blower 50 is preferably in the form of a two or three lobed rotaryblower coupled in direct drive fashion to an electric motor. In oneembodiment the electric motor is a five horsepower three phase motor andthe rotary blower is a two or three lobed blower and can deliverapproximately one hundred cubic feet per minute when operating atatmospheric pressure. This rotary blower is also preferably configuredto have acceptable performance when drawing a vacuum on the adsorber bed20.

The lobes of the rotary blower are preferably configured so that theyare of approximately similar efficiency in moving gases through theblower 50 between the entry 54 and the discharge 56 in either direction.In one form, the lobes are thus symmetrical in form so that they act onthe air similarly in both directions of rotation for the blower 50.

The blower 50 is preferably substantially of a positive displacementtype so that it maintains an adequate performance when drawing a vacuumon the adsorber bed 20 so that nitrogen can be effectively desorbed fromthe adsorber material in the adsorber bed 20 when the blower 50 isoperating in a reverse direction to pull nitrogen out of the adsorberbed 20 and deliver the nitrogen out of the entry 54.

Most preferably, the blower 50 is coupled in direct drive fashion to theelectric motor (or though a gear box). Most preferably, the electricmotor is a three phase alternating current motor which can easily bereversed by reversing two of the phases. In this way, the controller 60need merely reverse two poles of the three phase motor. In an otherembodiment a direct current, permanent magnet may be used wherein thedirection of the rotation can be reversed by reversing the polaritywhich in turn will reverse the rotation of the blower. Almost all threephase electric motors are capable of being reversed as above. Directcurrent motors are also readily available from many manufacturers whichreverse their rotation direction by changing polarity.

Other types of pumps could alternatively be utilized for drawing airinto the adsorber bed 20 and pulling nitrogen out of the adsorber bed 20for the oxygen separator 10. For instance, such a pump could be apositive displacement pump, such as a piston pump or a peristaltic pump.Other forms of positive displacement pumps could also be utilizedincluding gerotor pumps, gear pumps, etc. Other forms of pumps ratherthan strictly positive displacement pumps could also be selected, suchas centrifugal pumps or axial flow pumps. The most efficient scheme forpumping the air into the system and exhausting the bed depends on therequirements of the final user.

With continuing reference to FIG. 1, details of the controller 60 aredescribed according to a preferred embodiment. The controller 60 isshown as a separate unit coupled to the blower 50 (or other pump)through a blower signal line 62 and coupled to the valve 30 through avalve signal line 64. The controller 60 could in fact be integrated intothe valve 30 or integrated into the blower 50 (or other pump) or beprovided as a standalone unit such as depicted in FIG. 1. It is alsounderstood that the controller 60 could be split into two (or more)separate devices, either separate from the blower 50 and valve 30 orintegrated into both the blower 50 and valve 30.

The controller 60 provides the basic function of controlling a directionin which the blower 50 is operating and whether the valve 30 is open orclosed. Control systems have been used which simply time the cycle. Moreoften, the controller is configured to react to pressure or some otherinput.

A preferred sequence for directional control of the blower 50 andopening and closing of the valve 30 are described in detail below. Thecontroller 60 could be in the form of a programmable logic device orcould be in the form of an application specific integrated circuit, orcould be in the form of a CPU of a special purpose computer or a generalpurpose personal computer or other computing device. The controller 60could be configured to have operating parameters set at a centralcontrolled location, such as during manufacture, or could be configuredto allow for programming in the field before and/or during operation.

In use and operation, and with particular reference to FIG. 1, detailsof the operation of the oxygen separator 10 of the prior art aredescribed. It will be understood that the separator 10 would operatesimilarly when separating other gases than when separating oxygen fromair, and the operation as an oxygen separator 10 is provided merely asone example.

Initially, the system 10 is configured with the valve 30 closed and theblower 50 (or other pump) is caused to rotate in a direction drivinggases out of the adsorber bed 20 (along arrow E). This is the vacuumcycle used to desorb nitrogen out of the beads in the bed 20. Inparticular, the blower 50 rotates to cause gases to be pulled into theentry 54 (along arrow E). This gas is removed from the bed 20 by theblower 50 and caused to pass through the discharge 54 away from theadsorber bed 20 along arrow F and to the surrounding atmosphere.

Nitrogen (or other undesirable gas) is adsorbed by the adsorber materialwithin the adsorber bed 20. Most typically, the adsorber material alsoadsorbs water vapor and carbon dioxide, as well as potentially traceamounts of other gases, including pollutants.

During the last portion of the vacuum cycle valve 30 is opened to allowa small amount of the contents of the buffer tank to be introduced intothe adsorber bed. This step is called the “purge phase.” The purge phaseis used to purge nitrogen (as well as some carbon dioxide and water outof plumbing lines and free space between the valve 30 and the blower 50,but not appreciably out into the surrounding atmosphere. This shortpurge phase is typically timed to match an amount calculated ordetermined by experiment, but could also be ended based on sensorreadings. This purge phase ends the vacuum cycle and precedes theadsorption cycle to follow.

The blower is then reversed to commence the adsorption cycle. Air isdrawn into the blower at the inlet 54 port of the blower 50 (in thedirection shown by arrow A). The air flows (along arrow B) into theadsorber bed 20 where nitrogen, carbon dioxide, and water arepreferentially adsorbed. The gas not adsorbed in the adsorber bed(normally a mixture of oxygen and argon) is passed through valve 30 intothe buffer tank 40.

The adsorber bed 20 might also adsorb oxygen to some extent. However,the adsorber material is selected so that it preferentially adsorbsnitrogen more than oxygen. Due to the presence of the adsorber materialwithin the adsorber bed 20, substantially only oxygen (or otherdesirable gas) can leave the adsorber bed 20 through the outlet 26.Typically, argon also remains with the oxygen. Because air isapproximately 1% argon and approximately 20% oxygen, this twenty to oneratio typically causes the gases being discharged from the adsorber bed20 at the outlet 26 to be approximately 95% oxygen and 5% argon.

Because the valve 30 is opened, this oxygen can flow (along arrow C)through the valve 30 and into the buffer tank 40. The buffer tank 40 isthus charged with oxygen. If oxygen is desired, it can be dischargedfrom the buffer tank 40 output 46 (along arrow D). The adsorber materialwithin the adsorber bed 20 eventually becomes saturated with nitrogenand other compounds, such as water vapor and carbon dioxide. The pointof such saturation can be calculated in advance and calibrated into theseparator 10. Alternatively, a sensor can be provided, such as along theline 32 adjacent the valve 30, to sense for nitrogen or othercontaminants within what should be substantially only oxygen and argon.Such a sensor can cause the system to detect such saturation of theadsorbent material within the adsorber bed 20 and thus change the modeof operation of the oxygen separator 10 from the adsorption cycle to thevacuum cycle. Other sensors to trigger the change could be pressuresensors or volumetric flow rater sensors either alone or in combinationwith a clock or a calibration table. The goal is to prevent nitrogen orother contaminates from passing the valve 30 after adsorption bed 20saturation.

When such saturation has either been sensed as occurring or predicted tooccur, the separator 10 changes operation modes by closing the valve 30.Then the blower 50 (or other pump) reverses its direction of operation.For instance, the controller 60 can reverse two of the three phases of athree phase electric motor coupled to the blower. The blower 50 is thencaused to turn in an opposite direction and begins pulling gas (alongarrow E) out of the adsorber bed 20 and into the blower 50 from thedischarge 56 and out of the blower 50 through the entry 54 and out intoa surrounding environment, as a repeat of the vacuum cycle describedabove.

The controller 60 can be programmed with a typical amount of timerequired to effectively desorb the nitrogen from the adsorbent materialwithin the adsorber bed 20. Normally, the controller 60 senses athreshold low pressure in the adsorber bed 20. The system operation thencontinues as described above with a short purge phase followed by returnto the desorption cycle.

This operating sequence for the oxygen separator 10 can repeat itselfpotentially indefinitely. When the buffer tank 40 becomes full (orvessels being filled from the buffer tank 40 are full), an appropriatesensor associated with the buffer tank 40 can indicate that it is fulland shut off the oxygen separator 10. As further amounts of oxygen aresensed as being needed, such as by a drop in pressure in the buffer tank40, a signal can be sent to the controller 60 to again cause the systemto commence operation.

With this invention a modified air separation unit 110 implements amodification of the prior art single bed reversing blower (SBRB) vacuumswing adsorption (VSA) oxygen separator 10 through the air separationunit 110 of this invention and the driving system 210 described in moredetail below. The SBRB VSA air separation unit (ASU) 110 is modified inthis exemplary ASU 110 to include a purge recovery tank 160. Many otherportions of the ASU 110 have analogs in the prior art SBRB VSAtechnology such as that shown in FIG. 1.

In essence, and with particular reference to FIG. 2, basic details ofthe ASU 110 are described, according to a preferred embodiment with theASU 110 also typically including many of the details of the oxygenseparator 10 as described above. A single adsorber vessel 120 is fed byan intake 130 which supplies air to the vessel 120. Downstream of thevessel 120, an O₂ supply line 125 leads to an O₂ process tank 140 whichis optionally provided to contain excess O₂ before it is utilized byequipment and/or for processes downstream of the O₂ process tank 140. Areversible blower 150 is interposed between the adsorber vessel 120 andthe intake 130. A purge recovery tank 160 is coupled to the O₂ supplyline 125 downstream of the vessel 120, preferably through a controlvalve 165 to control whether the purge recovery tank 160 is open orclosed. A compressor 170 is preferably provided downstream of the O₂process tank which can control pressure of O₂ supplied from the ASU 110.

More specifically, and with continuing reference to FIG. 2, specificdetails of the ASU 110 are described. The single adsorber vessel 120extends between an inlet 122 and an outlet 124, with the inlet 122defining a side of the vessel 120 closest to the intake 130 and theoutlet 124 on a side of the vessel 120 opposite the inlet 122. Thisvessel 120 can have any of a variety of configurations. While thisvessel 120 is described as a single adsorber vessel 120, it isconceivable that a manifold upstream and downstream of the singleadsorber vessel 120 could be provided so that multiple vessels 120 couldbe provided in parallel, but operating in unison so that the ASU 110 isstill functioning as a single bed reversing blower (SBRB) system butwith optionally additional vessels 120 merely to adjust size of thevessel 120.

The vessel 120 contains an adsorption material which preferentiallyadsorbs N₂ over O₂. This material is typically provided in the form ofbeads or other solid media which allow for gas to flow about the solidmedia as the gas extends from the inlet 122 to the outlet 124, and pastsurfaces of the adsorption material. Surfaces of the adsorption materialadsorb nitrogen thereon, allowing O₂ to pass through the vessel 120.Typically, the material within the vessel 120 also adsorbs water vaporand various other gases, while typically argon within the air is notadsorbed but passes out of the vessel 120 along with the oxygen. Thevessel 120 includes a container wall which is sufficiently strong sothat it can maintain its volume when experiencing pressures ranging fromnear vacuum at a low end to approximately atmospheric (but potentiallyslightly higher than atmospheric pressure) at a high end.

The intake 130 in a simplest form merely includes an opening which isopen to a surrounding atmosphere for intake of air into the ASU 110. Inthe embodiment depicted, the intake 130 can include some form of filterelement, such as a particulate filter and includes an air port 132spaced from a purge port 134. A valve within the intake 130 causes airto be drawn in through the air port 132 when the blower 150 is drawingair into the vessel 120, and the purge port 134 discharges gas(including mostly N₂) when the blower 150 has reversed and is pullinggas out of the vessel 120. The purge port 134 is preferably spaced fromthe air port 132 to minimize the potential for nitrogen exhaust to findits way back into the air port 132. If desired, the purge port 134 canlead to other equipment such as nitrogen recovery equipment.

Regions downstream of the vessel 120 are together generally referred tocollectively as the O₂ output in that gas of mostly O₂ remains in theseportions of the ASU 110. The O₂ process tank 140 could be avoided insystems where oxygen is used as it is produced or where discharge ofexcess O₂ beyond that utilized by the equipment downstream of the ASU110 can merely be discharged to atmosphere, or can be avoided in systemswhere downstream equipment from the ASU 110 itself includes appropriatevolume, such as in the form of tanks or other equipment so that the O₂process tank 140 is not needed. However, typically an O₂ process tank140 is provided to hold excess O₂ produced when the reversible blower150 is driving air into the vessel 120 and the ASU 110 is producing O₂,so that when the blower 150 reverses and the vessel 120 is in recoverymode and discharging nitrogen therefrom, O₂ can continue to be suppliedfrom the O₂ process tank 140 to supply downstream oxygen utilizingequipment (FIG. 4).

Most preferably, a product check valve 145 is provided upstream of theO₂ process tank 140. This check valve 145 acts to keep pressurizedoxygen within the O₂ process tank 140 and preventing back flow of oxygenback toward the vessel 120. This product check valve 145 also providesone form of valve within the O₂ supply line 125 which the reversibleblower 150 works against so that an at least partial vacuum can be drawnon the vessel 120, without significant leakage of any gases into thevessel 120 from the O₂ supply line 125. Such a vacuum is needed to allowfor recovery of the material within the vessel 120 by causing thematerial to give up the N₂ and return to a state where it is ready toagain preferentially adsorb N₂ and supply O₂ to the O₂ process tank 140.The O₂ process tank 140 includes an inlet 142 opposite an outlet 144with the inlet 142 adjacent to the product check valve 145 and theoutlet 144 leading further into equipment downstream of the ASU 110which utilize oxygen.

The reversible blower 150 includes an inlet 152 on a side of thereversible blower 150 closest to the intake 130 and an outlet 154 on aside of the reversible blower 150 opposite the inlet 152. Thisreversible blower 150 is preferably a positive displacement pump, mosttypically of a rotary lobe variety which can both efficiently blow airthrough the vessel 120 to produce oxygen, but also effectively draw avacuum on the vessel 120 when reversed. The motor coupled to the rotarylobe prime mover of the reversible blower 150 is most preferably a typeof electric motor which can readily be reversed in direction, such as byreversing a polarity of an electric field associated with the electricmotor, or some other type of electric motor which can be readilyreversed in the direction that it is operating with a minimum of stresson the equipment associated with the reversible blower 150. Typically, acontroller is coupled to the reversible blower 150 which sends a signalat an appropriate time to the reversible blower 150 to cause it toreverse from pushing air into the vessel 120 to pulling gas out of thevessel 120.

The purge recovery tank 160 is preferably provided with an openingthereinto coupled to the O₂ supply line 125, preferably at a junction162 between the outlet 124 of the vessel 120 and the product check valve145. As an alternative, the purge recovery tank 150 can be coupleddirectly to the adsorber vessel 120 typically at a portion of theadsorber vessel 120 on a side of the vessel 120 opposite the inlet 122.

A control valve 165 is interposed between the tank 160 and the O₂ supplyline 125. Alternatively, this control valve 167 can be interposedbetween the tank 160 and the vessel 120. In either configuration, thecontrol valve 165, 167 transitions from a closed state where the purgerecovery tank 160 is isolated from the O₂ supply line 125 and theadsorber vessel 120 and an open state where the purge recovery tank 160is open to the O₂ supply line 125 and/or adsorber vessel 120. Thecontrol valve 165, 167 is typically coupled to a servo motor so that itis in the form of a servo valve (SV).

The control valve 165 is coupled to a controller which can be coupled toor the same as the controller associated with the reversible blower 150,so that opening and closing of the purge recovery tank 160 occurs in asynchronized fashion with reversing of the reversible blower 150. Ifdesired, such a controller or group of controllers can also be coupledto sensors such as a nitrogen sensor which can detect trace amounts ofN₂ downstream of the vessel 120 and indicative that the material withinthe vessel 120 is approaching saturation and the need to enter arecovery phase by reversing the reversible blower 150 and drawingnitrogen out of the vessel 120 through drawing a vacuum within thevessel 120. The controller can optionally include a clock and reversethe blower (and open/close the valve 165, 167) after set amounts of timehave passed.

The compressor 170 is optionally provided downstream of the O₂ supplyline 125 and downstream of any O₂ process tank 140. Preferably acompressor check valve 175 is provided upstream of the compressor 170.The compressor 170 allows for control of a pressure desired for O₂supplied from the ASU 110. The compressor check valve 175 assists inkeeping O₂ downstream of the compressor 170 from backing up into the ASU110.

With particular reference to FIGS. 3-5, general steps in operation ofthe ASU 110 are described. FIG. 3 depicts a feed step for the ASU 110.In this step the reversible blower 150 draws air from the air intake 130through the air port 132, along arrow G. The blower 150 drives air alongarrow H into the vessel 120. The air passes through the vessel 120(along arrow I) where nitrogen is selectively adsorbed. Gas of mostly O₂flows out of the vessel 120 (along arrow J) and within the O₂ supplyline 125. The control valve 165 of the purge recovery tank 160 is closedduring the beginning of the feed step so that O₂ flow continues past thejunction 162 and through the O₂ supply line 125 (along arrow K). Theoxygen then passes through the product check valve 145 and into the O₂process tank 140 (along arrow L). Further, the O₂ can flow through thecompressor check valve 175 and through the compressor 170 for dischargefrom the ASU 110 (along arrow M).

Such a feed step (as depicted in FIG. 3) continues as long as thematerial within the vessel 120 has excess capacity for adsorption ofnitrogen. When this adsorption material within the vessel 120 becomessaturated with nitrogen, the ASU 110 needs to prepare for recharging theadsorption material within the vessel 120. To detect that suchrecharging/restoration of the material within the vessel 120 is needed,the ASU 110 can follow a timing circuit or follow gas flow valves whichmeasure an amount of gas flow, or can include a nitrogen sensor or othersensor downstream of the vessel 120 which indicate that the gasdownstream of the vessel 120 is indicative thatrecharging/reconditioning of the material within the vessel 120 isneeded.

Preparation for recovery of the material within the vessel 120 can occurin a couple of slightly different but closely related ways. In oneembodiment, such preparation begins by opening of the control valve 165(or valve 167). The interior of the pressure recovery tank 160preferably has pressure below atmospheric pressure so that gas of mostlyoxygen (but with perhaps some nitrogen present) flows quickly into thepurge recovery tank 160 through the control valve 165.

When the purge recovery tank 160 is full, or when the purge recoverytank 160 is achieving a fill level which is sufficiently great tosatisfy its purposes in purge recovery for the vessel 120, the controlvalve 165 is closed. The purge recovery tank 160 thus contains and holdsa charge of mostly O₂ (but typically with some N₂ and other contaminatespresent) as a purge charge which can be at near atmospheric pressure, orconceivably above atmospheric pressure if pressure downstream of thevessel 120 is above atmospheric pressure.

The reversible blower 150 is instructed to reverse so that air is nolonger driven into the vessel 120, but the blower 150 reverses and gasesbegin to be pulled out of the vessel 120, through the reversible blower150 and back to the intake 130. The precise moment of beginningreversing of the reversible blower 150 could be before the control valve165 associated with the purge recovery tank 160 has closed, or could beat the same time that the control valve 165 closes, or could be slightlyafter the control valve 165 closes. The reversible blower 150 typicallytakes some time to stop moving in a forward direction and then beginmoving in a reverse direction. This slow down to zero velocity and speedup in a reverse direction also define a time period which can be duringwhich the control valve 165 closes or immediately before or immediatelyafter the control valve 165 closes.

The reversible blower 150 then operates in a reverse direction drawing avacuum on the adsorber vessel 120 and portions of the O₂ supply line 125between the adsorber vessel 120 and the product check valve 145 or othervalve on the O₂ supply line 125 which resists the draw of vacuum withinthe O₂ supply line 125. Pressure is thus reduced within the O₂ supplyline 125 and the adsorber vessel 120. Gas flow through the vessel 120occurs along arrow R of FIG. 5. As the pressure is reduced within theadsorber vessel, the ability of the material within the vessel 120 tohold N₂ decreases. N₂ is thus released from the adsorber material andflows, along arrow S (FIG. 5) through the reversible blower 150 and outof the purge port 134 of the intake 130 (along arrow T of FIG. 5). Aftera sufficient amount of time and sufficiently low pressure is achievedwithin the vessel 120 to satisfactorily allow the material within thevessel 120 to recover, the ASU 110 then undergoes preparation forre-reversing the reversible blower 150 and returning the ASU 110 back tothe feed mode (FIG. 3). This preparation typically initially involvesopening of the control valve 165 (or valve 167) associated with thepurge recovery tank 160. The mostly O₂ (with some N₂) gas that has beenstored therein is thus released through the control valve 165 and intothe O₂ supply line 125 (or directly into the vessel 120 through thevalve 167 of FIG. 2).

This purge of mostly O₂ with other gases into the low pressure vessel120 allows for pressure within the vessel 120 to be quickly restored andalso for the low quality purge gas which contains some N₂ and othercontaminant gases therein to again contact the adsorption materialwithin the vessel 120 for removal of N₂ and other contaminantstherefrom. Such purge flow is generally depicted by arrow P and also bearrow Q for return back into the adsorber vessel 120 (FIG. 5).

The vessel 120 has thus been fully prepared for returning back to thefeed step. The reversible blower 150 can then be re-reversed to againdrive airflow (along arrow H of FIG. 3) from the intake 130 (along arrowG) and through the vessel 120 (along arrow I). The control valve 165with the purge recovery tank 160 can be closed just before thereversible blower 150 re-reverses, at the same time that the reversibleblower 150 re-reverses, or shortly after the reversible blower 150re-reverses.

Various factors such as the volume of gas which can reside within thevarious lines adjacent to the purge recovery tank 160 and whether theASU 110 is to be optimized for O₂ purity, energy efficiency, orproduction rate, can be factored into determining precisely when thecontrol valve 165 (or 167) should be returned to its closed state.Similar optimization can occur when determining when to initially openthe control valve 165 and also when to initially close the control valve165. The control valve 165 is re-closed so that it maintains a vacuumtherein to make the purge recovery tank 160 most effective when it isagain utilized in the next iteration of the cycle performed by the ASU110.

With further reference to FIG. 5, details of a load following system 210are disclosed in a preferred embodiment, which allows the air separationunit 110 or similar adsorption based air separation units (such assystem 10 of FIG. 1) to be controlled to maintain efficient and reliableoperation even when O₂ demand is variable, such as to facilitate aturndown ratio of fifty percent, seventy-five percent or more. The loadfollowing system 210 includes a controller 220 which is coupled to atleast one pressure sensor 230 located in an O₂ storage region downstreamof the adsorption bed, depicted by the single adsorber vessel 120. Mostpreferably, a second pressure sensor 240 is also provided furtherdownstream within the O₂ storage region and downstream of the adsorbervessel 120.

This O₂ storage region 270 can generally be considered to include O₂flow lines downstream of the adsorber vessel 120, any purge recoverytank 160, an O₂ process tank 140 or other buffer tank, a compressor 170and various O₂ handling lines therebetween and downstream thereof. Thesecond pressure sensor 240 is preferably located downstream of thecompressor 170, and along a product line 244 through which O₂ isdischarged from the air separation unit 110.

The first pressure sensor 230 includes a data path 232 which feedspressure sensor data to the controller 220. The second pressure sensor240 includes a data path 242 supplying second pressure sensor data fromthe second pressure sensor 240 to the controller 220. The controller 220outputs control signals including a blower control signal 250 and acompressor control signal 260. While in simplest forms of this inventiona single pressure sensor somewhere within the O₂ storage could be fed tothe controller 220 and a single control signal, such as the blowercontrol signal 250, could be utilized, most preferably the controller220 receives at least two pressure sensor signals at two separatelocations within the O₂ storage 270 downstream of the adsorber vessel120, and supplies two separate control signals 250, 260 to control flowrates within the reversing blower air separation unit, such as the SBRBVSA air separation unit 110.

With particular reference to FIG. 6, exemplary details of sensedpressure at the two sensor 230, 240 locations are depicted along withpower level control signals generated by the controller 220 in response.FIG. 6 is a graph of power consumption and sensed pressure versus time.Power consumption is actually power consumed by the reversible blowerand also power consumed by the compressor 170. While these elements havesignificantly differing power consumptions, power consumption isdepicted as a ratio of “power used” to “full power.” Thus, the powerconsumption values appear to be similar to each other. As an example,the reversible blower might have a full power consumption rate of tenkilowatts. When the blower is being powered at seven kilowatts a valueof 0.7 would be depicted in the graph of FIG. 6. On the same graph, ifthe compressor has a full power rate of one kilowatt and is currentlyoperating at seven hundred watts, it would also display a powerconsumption of 0.7 in the graph of FIG. 6.

Similarly, the pressure data graphed on the common graph is not graphedin terms of actual pressure measured, but rather relative to a “pressuregoal” also referred to as a set point for a typical pressure to besensed at the first and second locations. This set point is not amaximum allowable pressure, but rather an optimal pressure or some otherarbitrary pressure value, with the system capable of handling pressurehigher than that of the pressure goal or lower than that of the pressuregoal. In one embodiment, an acceptable range of pressures could beutilized instead, with the set point being a midpoint in this range orsome other point in this range. Again, the first and second pressuresare typically different from each other (although they could besimilar), but are normalized relative to their set points. For instance,if the first pressure set point is 10 psig and the sensed pressure is7.5 psig, a value of twenty-five percent low would be graphed on thegraph of FIG. 6. Correspondingly, if the pressure set point at thesecond location is 5 psig and the actual pressure sensed is 3.75 psig,the pressure graphed for the second location would also be at“twenty-five percent low.”

Study of the graph of FIG. 6 indicates how sensed pressure at the firstlocation and the second location might vary somewhat in a particularinstance. Pressure values depicted herein are pressure at a process tank140 (or other buffer tank) and pressure at an output/supply, such as atthe product line 244. Factors influencing pressure at these locationsinclude demand for O₂. Because the output/supply pressure is closest tothe source of the O₂ load, the output/supply pressure is likely to bemost influenced by O₂ demand. Other factors which influence pressure atthe output/supply include the current flow rate of O₂ being deliveredfrom the compressor, and to some extent whether the SBRB VSA airseparation unit 110 is currently operating in a feed mode or in arecovery mode, and the fullness of the process tank 140. Pressure in theprocess tank 140 at the second location is influenced to some extent byO₂ demand, but because it is closer to the adsorber vessel 120 andreversible blower 150, it is more responsive to whether the airseparation unit 110 is currently in a feed mode, in a recovery mode, ina purge mode, or in some transition therebetween.

To maintain reliable operation, the blower power control signal actingin response to process tank pressure and the compressor power controlsignal operating responsive to output/supply pressure are two separatecontrol loops having different time intervals. The time intervals areelapsed time between when the control loop sends a new signal to theblower or compressor to have power consumption altered. In the exampledepicted in FIG. 6, the compressor power control has a time constant onthe order of one second while the blower power control signal has a timeconstant on the order of one minute. In the graph of FIG. 6, the blowerpower level thus makes adjustments every minute, but otherwise maintainsrelatively constant power consumption. In contrast, the compressor powerappears to be changing continuously in that it is changing approximatelyonce every second.

When a time constant is referred to as approximately one second orapproximately one minute, it is contemplated that such an approximatevalue might be up to five times greater or lesser than these values. Forinstance, with regard to a time constant of approximately one second, arange between 0.2 seconds and five seconds is contemplated. For a timeconstant of approximately one minute, a range of between 0.2 minutes andfive minutes is contemplated.

By keeping these time constants distinct from each other a variety ofbenefits are provided. First, excessive overreaction of the reversibleblower 150 to sense demand can cause the VSA ASU 110 to overreact andslow the separation process down too quickly and cause a loss in purityof O₂. If the time constants are too slow, the potential for a rapiddrop in demand can result in O₂ needing to be released from anoverpressure port before the load following system 210 can effectivelyrespond and reduce a rate of production. Furthermore, keeping the timeconstants similar has the propensity for the blower to follow thecompressor rather than to follow actual demand. The potential forinstability within the control system is thus increased. Suchinstability can require multiple changes in power consumption which areunnecessary, or for the control system to fail altogether and fall backonto override shut down circuitry or other circuitry which have thepotential to take the ASU 110 offline.

Most preferably, motors driving the compressor 170 and blower 150 arevariable frequency drives. Such drive motors can have their powerconsumption readily adjusted and corresponding flow rates can also bereadily adjusted, while efficiency can be maintained. Because the blower150 and compressor 170 are each preferably positive displacement flowinducing devices, the variable frequency drive motors can merely havetheir output shaft rotation rates modified, and flow rates arecorrespondingly modified, while efficiency is maintained to a greatextent. For instance, reduction of flow rate by fifty percent isaccompanied by a fifty percent lesser draw of power. Thus, efficientoperation can be maintained even when a fifty percent or seventy-fivepercent or more turn down ratio is required to meet changes in demand.FIG. 7 depicts a graph of power consumption versus O₂ production andillustrating how O₂ production reduction can occur with closelycorresponding power consumption reduction, maintaining efficiency.

In many O₂ supply installations it is desirable to match supply of O₂with the expected demand. Furthermore, there are benefits in utilizingstandardized units, rather than having a large number of differentlysized units grouped together. This can lead to suboptimal installationsin some circumstances. For instance, if a facility requires sixty tonsper day (TPD) of oxygen, and SBRB VSA air separation units are providedwhich have each unit supplying forty TPD, one would need to provide twoair separation units. However, the system would be capable of producingmore than it typically demanded. While it is possible to have one of theunits operate part-time, or to configure the units so that they sharethe duty cycle and alternate in shutting down, it is also beneficial ifthe units can follow the load and have a turndown ratio while stillmaintaining efficiency. Thus, with this invention a sixty TPD demand canbe met with two forty TPD units, with one unit operating at fullcapacity and the other unit operating at a turndown ratio of fiftypercent. As demand fluctuates upwardly and downwardly, this turndownratio can similarly be adjusted upwardly and downwardly, following theload. As an alternative, each of the units could be configured similarlyto have a high turndown ratio while maintaining efficiency, so that theycould each operate at approximately seventy-five percent of capacitywhen normal demand is experienced, and can act together or alternate inacting to meet demand changes utilizing the invention disclosed herein.

This disclosure is provided to reveal a preferred embodiment of theinvention and a best mode for practicing the invention. Having thusdescribed the invention in this way, it should be apparent that variousdifferent modifications can be made to the preferred embodiment withoutdeparting from the scope and spirit of this disclosure. When structuresare identified as a means to perform a function, the identification isintended to include all structures which can perform the functionspecified.

What is claimed is:
 1. An air separation process with load sensing andload following, including the steps of: sensing at least one pressurewithin a single adsorption bed air separation system having an air inletupstream from a reversing blower and an O₂ storage downstream from theadsorption bed; said sensing step detecting pressure magnitude at afirst location downstream from the reversing blower; altering a flowrate through the reversing blower responsive to the pressure of saidsensing step; and increasing a flow rate through the reversing blowerwhen pressure of said sensing step is below a set point; decreasing aflow rate through the reversing blower when the pressure of said sensingstep is above the set point; and including the step of further sensing asecond pressure at a second location downstream from the first location.2. The air separation process of claim 1 wherein said first location isat a buffer tank downstream from the adsorption bed.
 3. The airseparation process of claim 1 wherein a compressor is located within theO₂ storage downstream of the adsorption bed, the second locationdownstream from the compressor.
 4. The air separation process of claim 3including the further step of altering a flow rate through thecompressor responsive to the second pressure at the second location. 5.The air separation process of claim 4 wherein said altering a flow ratethrough the reversing blower step and said altering a flow rate throughthe compressor step each occur multiple times separated by passage of atleast one time interval.
 6. The air separation process of claim 5wherein a time interval for said altering a flow rate through thereversing blower step and a time interval for said altering a flow ratethrough the compressor step are different from each other.
 7. The airseparation process of claim 6 wherein the time interval for control ofthe reversing blower is longer than the time interval for control of thecompressor.
 8. The air separation process of claim 7 wherein the timeinterval for control of the reversing blower is approximately everyminute and the time interval for control of the compressor isapproximately every second.
 9. The air separation process of claim 5wherein said altering a flow rate through the reversing blower step andsaid altering a flow rate through the compressor step include the stepsof driving the reversing blower and the compressor with separatevariable frequency drives and controlling a rotational speed of thevariable frequency drives responsive to the pressure at the firstlocation and the pressure at the second location.
 10. An air separationmethod with load sensing and load following, including the steps of:sensing at least one pressure within a single adsorption bed airseparation system having an air inlet upstream from a reversing blowerand an O₂ storage downstream from the adsorption bed; said sensing stepdetecting pressure magnitude at a first location downstream from thereversing blower; altering a flow rate through the reversing blowerresponsive to the pressure of said sensing step; wherein said sensingstep includes the step of increasing a flow rate through the reversingblower when pressure is below a set point; decreasing a flow ratethrough the reversing blower when the pressure of said sensing step isabove the set point; wherein a compressor is located within the O₂storage downstream of the adsorption bed; and including the further stepof altering a flow rate through the compressor responsive to a secondpressure at a second location downstream from the compressor.
 11. Themethod of claim 10 wherein said altering a flow rate through thereversing blower step and said altering a flow rate through thecompressor step each occur multiple times separates by passage of atleast two time intervals; and wherein a time interval for said alteringa flow rate through the reversing blower step and a time interval forsaid altering a flow rate through the compressor step are different fromeach other.
 12. The method of claim 11 wherein said altering a flow ratethrough the reversing blower step and said altering a flow rate throughthe compressor step include the steps of driving the reversing blowerand the compressor with separate variable frequency drives andcontrolling a rotational speed of the variable frequency drivesresponsive to the pressure at the first location and the pressure at thesecond location.
 13. An air separation method with load sensing and loadfollowing, including the steps of: sensing at least one pressure withina single adsorption bed air separation system having an air inletupstream from a reversing blower and an O₂ storage downstream from theadsorption bed; said sensing step detecting pressure magnitude at afirst location downstream from the reversing blower; altering a flowrate through the reversing blower responsive to the pressure of saidsensing step; increasing a flow rate through the reversing blower whenpressure of said sensing step is below a set point; decreasing a flowrate through the reversing blower when the pressure of said sensing stepis above the set point; providing a compressor located within the O₂storage downstream of the adsorption bed; and further sensing a secondpressure at a second location downstream from the first location anddownstream from the compressor.
 14. The load following air separationmethod of claim 13 including the further step of altering a flow ratethrough the compressor responsive to the second pressure at the secondlocation; wherein said altering a flow rate through the reversing blowerstep and said altering a flow rate through the compressor step eachoccur multiple times separated by passage of at least two timeintervals; and wherein time intervals for said altering a flow ratethrough the reversing blower step and for said altering a flow ratethrough the compressor step are different from each other.
 15. The loadfollowing air separation method of claim 14 wherein the time intervalfor control of the reversing blower is longer than the time interval forcontrol of the compressor.
 16. The load following air separation methodof claim 15 wherein the time interval for control of the reversingblower is approximately every minute and the time interval for controlof the compressor is approximately every second.
 17. The load followingair separation method of claim 16 wherein said altering a flow ratethrough the reversing blower step and said altering a flow rate throughthe compressor step include the steps of driving the reversing blowerand the compressor with separate variable frequency drives andcontrolling a rotational speed of the variable frequency drivesresponsive to the pressure at the first location and the pressure at thesecond location.